There are many applications in which data signals are transmitted in a frequency division multiplexed manner, according to which different "channels" centered around different subcarrier frequencies are transmitted simultaneously. For example, this technique is used in telemetry applications for transmitting the various parameters connected with a missile in flight. The subcarrier frequencies commonly used for telemetry range between 400 Hz and 2.4 mHz with deviations (departures from the subcarrier frequency in accordance with the modulation signal) of plus or minus 1% to 40%. Accordingly, the highest frequency generally used for transmission of these signals is 2.4 mHz. This is consistent with the bandwidths of tape recorders used to record the telemetry signal for later demodulation and data processing.
The typical method of demodulating frequency division multiplexed signals is to provide a number of analog demodulators equal to the number of signals to be demodulated at a given time, each tuned to the center frequency of a single channel, and essentially demodulate each channel separately. The output signals, typically in the form of digital words representing the instantaneous amplitudes of the modulation signals, can then be processed simultaneously in a modern computer system. Such systems, involving a number of receiver circuits equal to the number of channels to be demodulated, tend to be relatively complex and costly, particularly when a large number of channels are to be simultaneously demodulated. Use of analog tuning elements in these circuits is also a source of continued complexity, particularly inasmuch as the circuits need to be tuned continually, are subject to drift over time, frequently introduce unavoidable distortion into the demodulation process, and require tuning when the subcarrier of a particular channel needs to be changed so that a different channel can be received.
It is known to use digital techniques for demodulating signals. Such techniques typically involve sampling the input signal at regular intervals to generate a series of digital words corresponding to the amplitude of the signal and mixing the signal with digital samples of sine and cosine signals of frequencies corresponding to the channel center frequency. The difference component comprises upper and lower side bands of the original subcarrier in quadrature. The quadrature components contain phase difference information corresponding to the original modulation signals. The original modulation signals can thus be retrieved by analysis of the phase information.
Typically the phase information can be treated as a vector in the complex plane. Prior techniques for measuring the angle of this vector and hence for recovering the phase information have involved use of look-up tables in which trigonometric values are stored. For example, U.S. Pat. No. 4,603,300 to Welles shows such a system, in which the tangent of the vector to one axis is calculated and the calculated value is then used to access a look-up table to determine the angle of the vector. However, in the event that the denominator of the tangent to be calculated is zero, this process involves division by zero with inappropriate results. Accordingly, it would be desirable to avoid this type of decoding process for measuring the angle of the vector to the real axis.
From time to time it is also desired that the input frequency division multiplexed signal be stored for later analysis. Typically, tape recorders are used to store the input signal. However, even the highly sophisticated tape recorders now in use are subject to some variation in running speed which can introduce distortion into a frequency modulated input signal. Accordingly, it would be desirable if means could be provided for ready compensation of any such distortion in the recorded signal.
Finally, it would be desirable to provide a frequency division multiplexed signal demodulator which can readily be reconfigured for operation with different channels at different subcarrier frequencies, which avoids use of analog circuit elements subject to drift, and which is readily controllable by an operator.